Agatized CORAL

Florida's Collectible State Stone

- RICHARD GROSS

Ancient coral reefs off the western coast of central and south Florida must have been beautiful when living. We know they were warm shallow seas, teeming with life.

Now, agatized coral geodes bring that beauty to the present. In 1979, Florida agatized coral was designated by the Florida Legislature as the official state stone. Besides being displayed by individuals and museums in their collections, the coral has lapidary uses.

HOW AGATIZED CORAL FORMED

Florida agatized corals are the skeletal remains of colonial animals, known as anthozoans, that died 25 to 38 million years ago. The actual skeletal structures are very easy to see in some specimens and can be used to identify the coral species.

The skeletons consist of limestone (CaCO3) and the coral becomes agatized when the coral voids are filled with silica-rich water and the limestone is replaced by chalcedony quartz (SiO2) and common opal. Chalcedony that is translucent and colored is called agate.

The replacement process takes millions of years and results in what geologists refer to as a pseudomorph – a crystal consisting of one mineral, but having the form of another which it has replaced. Occasionally, very small clear visible quartz crystals will form after the chalcedony layer; but, it is the richly colorful agate layer that we find so attractive.

IS AGATIZED CORAL A GEODE?

As far as geodes go, these wonders of an ancient ocean are atypical. While some are the round shape we generally see in most geodes, many have shapes that vary.